Shader "Builtin Skybox"
{
    Properties
    {
        _Tint ("Tint", Color) = (1,1,1,1) [Semantic(Tint)]
        _Exposure ("Exposure", Float) = 1.0 [Semantic(Exposure)]
        _Rotation ("Rotation", Float) = 0.0 [Semantic(Rotation)]
        _MainTex ("Panoramic", 2D) = "white" [Semantic(SkyboxPanoramicTexture)]
        _Tex ("Cubemap (HDR)", Cube) = "white" [Semantic(SkyboxTexture)]
    }
    HLSLINCLUDE
    cbuffer EnvironmentConstants
    {
        float4 gSkyboxTopColor;
        float4 gSkyboxHorizonColor;
        float4 gSkyboxBottomColor;
        float4 gCameraRightAndTanHalfFov;
        float4 gCameraUpAndAspect;
        float4 gCameraForwardAndUnused;
    };

    cbuffer MaterialConstants
    {
        float4 gSkyboxTintAndExposure;
        float4 gSkyboxRotationAndMode;
    };

    Texture2D SkyboxPanoramicTexture;
    TextureCube SkyboxTexture;
    SamplerState LinearClampSampler;

    struct VSOutput
    {
        float4 position : SV_POSITION;
        float2 ndc : TEXCOORD0;
    };

    static const float XC_PI = 3.14159265358979323846f;
    static const float XC_INV_PI = 0.31830988618379067154f;
    static const float XC_INV_TWO_PI = 0.15915494309189533577f;

    VSOutput MainVS(uint vertexId : SV_VertexID)
    {
        const float2 positions[3] = {
            float2(-1.0f, -1.0f),
            float2(-1.0f,  3.0f),
            float2( 3.0f, -1.0f)
        };

        VSOutput output;
        output.position = float4(positions[vertexId], 1.0f, 1.0f);
        output.ndc = positions[vertexId];
        return output;
    }

    float3 EvaluateProceduralSkybox(float3 viewRay)
    {
        const float vertical = clamp(viewRay.y, -1.0f, 1.0f);
        float3 color = gSkyboxHorizonColor.rgb;
        if (vertical >= 0.0f) {
            color = lerp(gSkyboxHorizonColor.rgb, gSkyboxTopColor.rgb, pow(saturate(vertical), 0.65f));
        } else {
            color = lerp(gSkyboxHorizonColor.rgb, gSkyboxBottomColor.rgb, pow(saturate(-vertical), 0.55f));
        }

        return color;
    }

    float3 RotateAroundY(float3 viewRay)
    {
        const float rotation = gSkyboxRotationAndMode.x;
        const float sinTheta = sin(rotation);
        const float cosTheta = cos(rotation);
        return normalize(float3(
            viewRay.x * cosTheta - viewRay.z * sinTheta,
            viewRay.y,
            viewRay.x * sinTheta + viewRay.z * cosTheta));
    }

    float2 ComputePanoramicUv(float3 viewRay)
    {
        const float3 rotatedRay = RotateAroundY(viewRay);
        const float u = frac(atan2(rotatedRay.z, rotatedRay.x) * XC_INV_TWO_PI + 0.5f);
        const float v = acos(clamp(rotatedRay.y, -1.0f, 1.0f)) * XC_INV_PI;
        return float2(u, saturate(v));
    }

    float4 MainPS(VSOutput input) : SV_TARGET
    {
        const float tanHalfFov = gCameraRightAndTanHalfFov.w;
        const float aspect = gCameraUpAndAspect.w;

        const float3 viewRay = normalize(
            gCameraForwardAndUnused.xyz +
            input.ndc.x * aspect * tanHalfFov * gCameraRightAndTanHalfFov.xyz +
            input.ndc.y * tanHalfFov * gCameraUpAndAspect.xyz);

        float3 color = EvaluateProceduralSkybox(viewRay);
        if (gSkyboxRotationAndMode.y > 1.5f) {
            color = SkyboxTexture.Sample(LinearClampSampler, RotateAroundY(viewRay)).rgb *
                    gSkyboxTintAndExposure.rgb *
                    gSkyboxTintAndExposure.w;
        } else if (gSkyboxRotationAndMode.y > 0.5f) {
            color = SkyboxPanoramicTexture.Sample(LinearClampSampler, ComputePanoramicUv(viewRay)).rgb *
                    gSkyboxTintAndExposure.rgb *
                    gSkyboxTintAndExposure.w;
        }

        return float4(color, 1.0f);
    }
    ENDHLSL
    SubShader
    {
        Pass
        {
            Name "Skybox"
            Tags { "LightMode" = "Skybox" }
            Cull Off
            ZWrite Off
            ZTest LEqual
            Blend Off
            HLSLPROGRAM
            #pragma target 4.5
            #pragma vertex MainVS
            #pragma fragment MainPS
            ENDHLSL
        }
    }
}